1. Field of the Invention
This invention concerns means and methods for blocking the rotation of marine propellers and, more particularly, apparatus and techniques for preventing rotation of propellers on outboard motors and outdrive units of inboard-outboard marine propulsion systems to facilitate removal and installation of such propellers.
2. Background Art
Outboard motors are manufactured in sizes ranging from about one horsepower to several hundred horsepower. Inboard-outboard propulsion systems have even higher power ratings. A common feature of both outboard motors and inboard-outboard propulsion systems is that the propeller is mounted on a submersible lower unit which extends into the water from the transom of the boat and which turns in response to the steering linkage to direct the thrust of the propeller partly to the port or starboard in order to steer the vessel.
The outdrive portion of an inboard-outboard propulsion system and the lower unit of an outboard motor are structurally similar in several respects. Both have a substantially horizontal anti-cavitation plate mounted above the propeller, both have a gear box to couple the substantially vertical internal drive shaft to the substantially horizontal propeller shaft, both have a skeg or other protrusion below the propeller to protect the propeller from obstacles on the bottom, and both have removable propellers. Frequently, both have an outlet for engine exhaust gases extending through the propeller hub, but this and other internal construction details of the lower unit portion of the outboard motor or inboard-outboard drive are not of interest to the present invention. As used herein, the words "lower unit" are intended to include both outboard motor lower units and inboard-outboard drive lower units which are submersible and have anti-cavitation plates extending over removable propellers.
The drive shaft of a typical lower unit passes through the axis of rotation of the propeller. The propeller is coupled to the drive shaft by a shearable pin or friction device so that the rotational energy of the drive shaft is transmitted to the propeller. Typically, the propeller is retained on the drive shaft by a washer and locking nut on the end of the drive shaft. In order to remove the propeller, it is necessary to unscrew the retaining nut or other propeller locking means. Depending on how long the nut has been mounted on the propeller shaft, a large torque may be required in order to break it loose. This is especially true with high horsepower units having large propellers and correspondingly large propeller shafts and retaining nuts.
In many situations, the internal gears of the lower unit cannot be used to immobilize the propeller shaft while one attempts to remove or tighten the retaining nut. Thus, a continuing need exists for improved means and methods for preventing the propeller from rotating while torque is applied to remove the propeller retaining nut. In the past it has been customary to wedge a tool or piece of wood between the propeller and the anti-cavitation plate, but such methods are not reliable, especially when the required torque is large. Slippage and injury often result.
A prior art attempt to solve this problem is described in U.S. Pat. No. 4,624,644 in which a propeller blocking unit is obtained by sliding a specially shaped hollow structure over the anti-cavitation plate. Because lower units from different manufacturers and of different horsepower rating have very different anti-cavitation plate designs, a service technician would be required to stock a substantial number of such blocking units of different shape and dimensions. This is not desirable.
Accordingly, it is an object of the present invention to provide an improved means and method for blocking the rotation of marine propellers on marine lower units to facilitate removal of the propeller retaining nut or other propeller retaining mechanism.
It is a further object of the present invention to provide such improved means and method in a simple and effective manner that is less prone to slippage than prior art methods.
It is a still further object of the present invention to provide such improved means which is adapted to fit on lower units of different manufacturers and of a wide range of power ratings.
It is an additional object of the present invention to provide such improved means of comparatively inexpensive materials that are light weight, durable and able to block the propeller against light torques.